This invention relates to the improved preparation of alkaline earth metal, aluminum-containing spinel compositions, particularly for use in the combusting of solid, sulfur-containing material in a manner to effect a reduction in the emission of sulfur oxides to the atmosphere. In one specific embodiment, the invention involves the catalytic cracking of sulfur-containing hydrocarbon feedstocks in a manner to effect a reduction in the amount of sulfur oxides emitted from the regeneration zone of a hydrocarbon catalytic cracking unit.
Typically, catalytic cracking of hydrocarbons takes place in a reaction zone at hydrocarbon cracking conditions to produce at least one hydrocarbon product and to cause carbonaceous material (coke) to be deposited on the catalyst. It has been reported that approximately 50% of the feed sulfur is converted to H.sub.2 S in the fluid bed catalytic cracking (FCC) reactor, 40% remains in the liquid products and about 4 to 10% is deposited on the catalyst. These amounts vary with the type of feed, rate of hydrocarbon recycle, steam stripping rate, the type of catalyst, reactor temperature, etc.
Sulfur-containing coke deposits tend to deactivate cracking catalyst. Cracking catalyst is advantageously continuously regenerated, by combustion with oxygen-containing gas in a regeneration zone, to low coke levels, typically below about 0.4% by weight, to perform satisfactorily when it is recycled to the reactor. In the regeneration zone, at least a portion of sulfur, along with carbon and hydrogen, which is deposited on the catalyst, is oxidized and leaves in the form of sulfur oxides (SO.sub.2 and SO.sub.3, hereinafter referred to as "SOx") along with substantial amounts of CO, CO.sub.2 and H.sub.2 O.
Considerable recent research effort has been directed to the reduction of sulfur oxide emissions from the regeneration zones of hydrocarbon catalytic cracking units. One technique involved circulating one or more metal oxides capable of associating with oxides of sulfur with the cracking catalyst inventory in the regeneration zone. When the particles containing associated oxides of sulfur are circulated to the reducing atmosphere of the cracking zone, the associated sulfur compounds are released as gaseous sulfur-bearing material such as hydrogen sulfide which is discharged with the products from the cracking zone and are in a form which can be readily handled in a typical facility, e.g., petroleum refinery. The metal reactant is regenerated to an active form, and is capable of further associating with the sulfur oxides when cycled to the regeneration zone.
Incorporation of Group II metal oxides on particles of cracking catalyst in such a process has been proposed (U.S. Pat. No. 3,835,031 to Bertolacini). In a related process described in U.S. Pat. No. 4,071,436 to Blanton, et al., discrete fluidizable alumina-containing particles are circulated through the cracking and regenerator zones along with physically separate particles of the active zeolitic cracking catalyst. The alumina particles pick up oxides of sulfur in the regenerator, forming at least one solid compound, including both sulfur and aluminum atoms. The sulfur atoms are released as volatiles, including hydrogen sulfide, in the cracking unit. U.S. Pat. No. 4,071,436 further discloses that 0.1 to 10 weight percent MgO and/or 0.1 to 5 weight percent Cr.sub.2 O.sub.3 are preferably present in the alumina-containing particles. Chromium is used to promote coke burnoff.
A metallic component, either incorporated into catalyst particles or present on any of a variety of "inert" supports, is exposed alternately to the oxidizing atmosphere of the regeneration zone of an fluid bed catalytic cracking unit (FCCU) and the reducing atmosphere of the cracking zone to reduce sulfur oxide emissions from regenerator gases in accordance with the teachings of U.S. Pat. Nos. 4,153,534 and 4,153,535 to Vasalos and Vasalos, et al., respectively. In Vasalos, et al., a metallic oxidation promoter such as platinum is also present when carbon monoxide emissions are to be reduced. These patents disclose nineteen different metallic components, including materials as diverse as alkaline earths, sodium, heavy metals and rare earth, as being suitable reactants for reducing emissions of oxides of sulfur. The metallic reactants that are especially preferred are sodium, mangnesium, manganese and copper. When used as the carrier for the metallic reactant, the supports that are used preferably have a surface area at least 50 square meters per gram. Examples of allegedly "inert" supports are silica, alumina and silica-alumina. The Vasalos and Vasalos, et al., patents further disclose that when certain metallic reactants (exemplified by oxides or iron, manganese or cerium) are employed to capture oxides of sulfur, such metallic components can be in the form of a finely divided fluidizable powder.
Similarly, a vast number of sorbents have been proposed for desulfurization of non-FCCU flue gases in zones outside the unit in which SOx is generated. In some such non-FCCU applications, the sorbents are regenerated in environments appreciably richer in hydrogen than the cracking zone of an FCC unit. Cerium oxide is one of fifteen adsorbents disclosed for flue gas desulfurization in a publication of Lowell, et al., "SELECTION OF METAL OXIDES FOR REMOVING SOx FROM FLUE GAS," Ind. Eng. Chemical Process Design Development, Vol. 10, Nov. 3, 1971. In U.S. Pat. No. 4,001,375 to Longo, cerium on an alumina support is used to absorb SO.sub.2 from non-FCCU flue gas streams or automobile exhaust at temperatures of 572.degree. to 1472.degree. F., preferably 932.degree. to 1100.degree. F. During regeneration the desorbed species is initially SO.sub.2 and H.sub.2 S along with excess reducing gases which can be used as feedstock for a Claus unit. The Longo patent is not concerned with reducing emissions from an FCC unit and the reducing emissions from an FCC unit and the reducing atmosphere employed in practice of this process differs significantly from the hydrocarbon-rich atmosphere in a catalytic cracker. Thus, a hydrocarbon cracking reaction zone is preferably operated in the substantial absence of added hydrogen while the presence of sweeping amounts of hydrogen gas is essential to the regeneration step in practice of the process of Longo.
D. W. DeBerry, et al., "RATES OF REACTION OF SO.sub.2 WITH METAL OXIDES," Canadian Journal of Chemical Engineering, 49, 781 (1971), reports that cerium oxide was found to form sulfates more rapidly than most of the other oxides tested. The temperatures used, however, were below 900.degree. F. and thus below those preferred for use in catalyst regenerators in FCC units.
Many commercial zeolitic FCC catalyst contain up to 4% rare earth oxide, the rare earth being used to stabilize the zeolite and provide increased activity. See, for example, U.S. Pat. No. 3,930,987 to Grand. The rare earths are most often used as mixtures of La.sub.2 O.sub.3, Pr.sub.6 O.sub.11, Nd.sub.2 O.sub.3 and others. Some catalyst is produced by using a lanthanum-rich mixture of rare earth. It has been found that the mere presence of rare earth in a zeolitic cracking catalyst will not necessarily reduce SOx emissions to an appreciable extent.
In accordance with the teachings of U.S. Pat. No. 3,923,092 to Gladrow, certain zeolitic catalyst compositions capable of being regenerated at a rate appreciably faster than prior art rare earth exchanged zeolitic catalyst compositions are produced by treating a previously rare earth exchanged zeolitic catalyst composition with a dilute solution containing cerium cations (or a mixture of rare earths rich in cerium). The final catalyst contain 0.5 to 4% cerium cations which are introduced to previously rare earth exchanged zeolitic catalyst particles prior to final filtering, rinsing and calcining. Cerium is described as an "oxidation promoter". There is not recognition or appreciation in the patent of the effect of the cerium impregnation of SOx stack emissions. Such impregnation of rare earth exchanged zeolitic catalyst particles is not always effective in producing modified catalysts having significant ability to bind oxides of sulfur in a FCC regenerator and release them in a FCC cracking reaction zone.
Thus, considerable amount of study and research effort has been directed to reducing oxide of sulfur emissions from various gaseous streams, including those from the stacks of the regenerators of FCC units. However, the results leave much to be desired. Many metallic compound shave been proposed as materials to pick up oxides of sulfur in FCC units (and other desulfurization applications) and a variety of supports, including particles of cracking catalysts and "inerts", have been suggested as carriers for active metallic reactants. Many of the proposed metallic reactants lose effectiveness when subjected to repeated cycling. Thus, when Group II metal oxides are impregnated on FCC catalysts or various supports, the activity of the Group II metals is rapidly reduced under the influence of the cyclic conditions. Discrete alumina particles, when combined with silica-containing catalyst particles and subjected to steam at elevated temperatures, e.g., those present in FCC unit regenerators, are of limited effectiveness in reducing SOx emissions. Incorporation of sufficient chromium on an alumina support to improve SOx sorption results in undesirably increased coke and gas production.
Commonly assigned U.S. patent applications, namely, U.S. application Ser. No. 494,602, filed May 16, 1983, and U.S. application Ser. No. 494,753, filed May 16, 1983, U.S. Pat. No. 4,469,589, relate to improved materials for reducing SOx emissions, incorporating, respectively, spinel compositions, preferably alkaline earth metal-containing spinels, and spinel compositions including at least one additional metal component. The specification of each of these patent applications is incorporated herein by reference.
Various methods have been described for the preparation of alkaline earth aluminate spinels, and particularly of magnesium aluminate spinels. According to the method disclosed in U.S. Pat. No. 2,992,191, the spinel can be formed by reacting, in an aqueous medium, a water-soluble magnesium inorganic salt and a water-soluble aluminum salt in which the aluminum is present in the anion. This patent does not teach controlling pH during the time the two salts are combined.
Another process for producing magnesium aluminate spinel is set forth in U.S. Pat. No. 3,791,992. This process includes adding a highly basic solution of an alkali metal aluminate to a solution of a soluble salt of magnesium with no control of pH during the addition, separating and washing the resulting precipitate; exchanging the washed precipitate with a solution of an ammonium compound to decrease the alkali metal content; followed by washing, drying, forming and calcination steps.
Further commonly assigned U.S. patent applications, namely, U.S. application Ser. No. 445,304, filed Nov. 29, 1982, U.S. application Ser. No. 445,305, filed Nov. 29, 1982, U.S. application Ser. No. 445,306, filed Nov. 29, 1982, and U.S. application Ser. No. 445,130, filed Nov. 29, 1982, U.S. Pat. No. 4,471,070 relate to novel process steps for the improved production of alkaline earth metal and aluminum-containing spinel compositions. The specification of each of these patent applications is incorporated herein by reference.
U.S. Pat. No. 4,428,827 teaches producing a sulfur acceptor solid containing magnesium, sodium and aluminum using a precipitating agent to cause the formation of highly insoluble magnesium and aluminum using a precipitate to cause the formation of a highly insoluble magnesium and aluminum salts which will remain stable in an alkaline solution.
Attention has been given to improved zeolite catalyst compositions of specific pore diameters prepared under conditions where a portion of the alkali metal ion component is replaced by a nitrogen base such as ammonium ion or a basic organic nitrogen compound. Rub, et al., U.S. Pat. No. 4,021,447 describes the preparation of ZSM-4 by synthesis in the presence of pyrrolidine or choline salts, rather than tetramethylammonium hydroxide or halide, to yield the same crystal structure. Improved stability is also claimed. Similarly, Plant, et al., U.S. Pat. No. 4,021,502 employs ammonium or alkyl ammonium salts in zeolite formation. Argauer, et al., U.S. Reissue Pat. No. 29,857 similarly prepares ZSM-5 catalysts. Rollmann, U.S. Pat. No. 4,148,713 employs tetrapropyl ammonium cations and Rankel, et al., U.S. Pat. No. 4,388,285 prepares ZSM-5 catalyst with the aid of complexes such as a metal phthalocyanin, iron cyclopentadienyl, and the like.
Daniels, et al., in "CATIONIC POLYMERS AS TEMPLATES IN ZEOLITE CRYSTALLIZATION", J. American Chemical Society, Vol. 100, Pages 3097-3100, May 15, 1978, describe their experience with certain organic polymers in forcing the crystallization of large-pore mordenite under conditions which would otherwise have led to small-pore zeolites. Only a small number of such polyelectrolytes were found to be effective. Haas, et al., in "PREPARATION OF METAL OXIDE GEL SPHERES WITH HEXAMETHYLENE TETRAMINE AS AN AMMONIA DONOR", Ind. Eng. Chem. Product Research Development, Vol. 22, No. 3, Pages 481-486, 1983, describes the use of such nitrogen complexes as a source for the slow release of ammonia in the precipitation of hydrous oxide gels which yield oxide spheres having high surface area and other desirable properties.
There remains a need for improved spinel catalyst components, exhibiting good SOx removal properties, and for improved processing in their manufacture.